The invention relates to preparation and uses of novel polymeric materials, polyimide amic acid salts (PIAAS). The use of these materials for the fabrication of fluid separation membranes is further disclosed.
Polyimides are high performance polymers with excellent mechanic and thermal properties. However, polyimides with particularly desirable mechanic and thermal characteristics generally have poor solubility in common organic solvents, such as dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), and chloroform. Therefore, polyimide articles are generally obtained from their soluble precursors by treating with heat or with chemical dehydrating agents. Three types of precursors have been used for the processing of polyimide materials, as shown in FIG. 1. They are polyamic acid (PA, I), polyamic acid ester (PAE, II) and polyamic acid salt (PAAS, III), respectively.
Polyamic acid (I) can decompose into an amine terminal radical and an acid anhydride terminal radical by depolymerization due to reaction equilibrium. The acid anhydride radical formed will react with water that can be introduced from outside or produced during the imidization and thus will be transformed into dicarboxylic acid that will no long react with amine terminal radical to form covalent bonds. As the result, molecular weight of the polyamic acid will decrease over time when stored. This in turn will effect the mechanical characteristics and the quality of the final polyimide article. The most frequently suggested method to overcome the depolymerization is to store the polyamic acid solution at low temperatures or to use the precursor shortly after its preparation.
Polyamic acid polymers have been used as the intermediate material for the fabrication of asymmetric and composite membranes. The formation of asymmetric membrane is often accomplished through a phase inversion process by contacting polymer solution (dope) with a non solvent. The preferred non solvent is frequently water. Polyamic acid is a hydrophilic polymer that will strongly associate with polar solvents, such as N,Nxe2x80x2-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), hexamethylphosphoramide (HMPA), N-acetyl-2-pyrrolidone, N,Nxe2x80x2-dimethylacetamide (DMAc) and the like, which are common solvents used for the preparation of polyamic acid solutions. Therefore, the formation of polyamic acid membranes by phase inversion process is essential any nonsolvent is very difficult because of the slow solvent exchange. For example, asymmetric polyimide membranes derived from pyromellitic dianhydride and 4,4xe2x80x2-oxydianiline (PMDA-ODA) required gelation in toluene for 24 hours followed by gellation in dioctyl sebacate for another 24 hours, as described by H. Ohya, T. Ichihara, T. Higashijima, Y. Negishi, xe2x80x9cMembranexe2x80x9d, 17(1), 42 (1992). Semenova et al. reported that asymmtric membranes from polyamic acid (PMDA-ODA) solutions can.not be prepared by phase inversion into ethanol, 1,4-dioxane, water and ethyleneglycol, due to membrane disintegration upon immersion, see S. I. Semenova, H. Ohya, T. Higashijima, and Y. Negishi, xe2x80x9cMembranexe2x80x9d, 17(3), 193 (1992).
U.S. Pat. No. 5,510,395 discloses the preparation of a polyimide microporous film from a polyamic acid casting solution formed in a volatile solvent composition by evaporation process. It is difficult to synthesize high molecular weight polyamic acid polymers in the disclosed solvent system. Hence, the mechanical properties of the resultant polyimide are compromised.
U.S. Pat. Nos. 5,725,769 and 5,753,008 disclose a process for the preparation of asymmetric solvent resistant membranes. In the first step the polyamic acid is synthesized in NMP solvent. Then a large amount of glycerin is added to the solution to form the membrane casting solution. The casting solution is then extruded through the orifice of a tube-in-orifice spinneret into a water bath to form the nascent hollow fiber. The polyamic acid hollow fiber is then converted into the polyimide hollow fiber by heat treatment. As pointed out previously, polyamic acid is highly hydrophilic, which makes the solvent exchange slow. Extended time is required to fully exchange the associated NMP solvent by soaking the nascent hollow fiber membrane in water to remove solvent. Any residual solvent can adversely effect membrane porosity during the subsequent heat treatment. However, extensive washing of hollow fibers with water tends to hydrolyze the polyamic acid polymer and the final hollow fiber formed can be brittle.
In U.S. Pat. No. 4,113,628 preparation of asymmetric polyimide hollow fiber membranes is disclosed from the corresponding polyamic acid solutions by quenching the nascent hollow fiber into selected chemical cyclization compositions, such as acetic anhydride and triethylamine in benzene solution. While, the phase separation and imidization processes take place simultaneously, the disclosed process requires the use of large amounts of organic solvents and thus is difficult for commercialization.
U.S. Pat. Nos. 4,440,643 and 5,141,642 disclose the preparation of composite polyimide gas separation membranes from polyamic acid precursors. However, fabrication of reproducible polyimide membranes from polyamic acid precursors is extremely difficult due to the sensitivity of polyamic acids towards temperature and moisture. Furthermore, some polyamic acids are not soluble in mild organic solvents, and all polyamic acids require harsh conditions to complete imidization. For example, temperatures as high as 300xc2x0 C. are generally required to complete imidization of polyamic acids by thermal treatment. The limited availability of solvent systems and the requirement for high imidization temperatures prohibits the application of polyamic acid precursors as the coating material for the fabrication of composite polyimide membranes when the preferred, readily available polymeric substrates, such as polysulfone are used. To maintain a high level of substrate porosity, the thermal imidization temperature must be lower than the glass transition temperature of the substrate polymer, and most of the commercially employed polymeric substrates have glass transition temperatures below 200xc2x0 C. (for example, the Tg of polysulfone is about 190xc2x0 C.).
G. A. Polotskaya et al. disclosed a polyamic acid membrane casting composition that exhibits a lower imidization temperature, see G. A. Polotskaya, et al., Polymer Science, Ser. B., Volume 38, pp281, 1996 (English translation). The composition is formed by combining a polyamic acid dissolved in dimethylacetamide (DMAc) with 0.5-2.0 equivalent amount of benzimidazole. The composition is used to form composite membranes by coating a poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) porous substrate saturated with high aliphatic hydrocarbons. The coating layer is converted to polyimide by heat treatment at 150xc2x0 C. The disclosed method, however, requires the use of a harsh, high boiling coating solvent. The solvent used is difficult to remove and can damage or destroy the porous substrate. As the result, the composite membrane formed had inferior gas permeation properties. Furthermore, the disclosed method can be applied only to a limited number of solvent resistant substrates.
Polyamic ester (II) is another polyimide precursor utilized extensively that has improved chemical stability and processing ability. The polyamic ester is obtained by esterification of the carboxylic acid. The ester formation prevents the depolymerization that effects the polyamic acid precursors. However, the synthesis of tetracarboxylic acid diester dichloride, an essential monomer for the synthesis of polyamic ester, is very difficult. The monomer is sensitive to moisture and tends to deactivate on prolonged storage.
U.S. Pat. No. 5,952,448 discloses a process for the preparation of partially imidized polyamic ester. The partially imidized polyamic ester is obtained by partial esterification of amic acid radicals of the polyamic acid with a base, such as potassium carbonate, and the esterification agent, such as alkyl halide, followed by imidization of the remaining amic acid radicals. The use of a strong inorganic base can, however, cause depolymerization of the polyamic acid that will effect the mechanic characteristics of polyimide polymer formed.
The temperature required for complete thermal imidization of polyamic ester is even higher than that required for polyamic acid precursors, see for example, Y. Charlie, et al. Polymer, Volume 36, Pages 1315-1320, 1995. The extremely high temperature required for the thermal imidization of polyamic ester is prohibitive for the fabrication of certain polyimide articles, such as porous membranes, from polyamic ester precursors, as the porosity of the precursory asymmetric or composite membrane is destroyed during the high temperature heating process.
Polyamic acid salt (III) precursors are formed by neutralization of the free carboxylic acid group of the polyamic acids with a base, such as a tertiary amine. U.S. Pat. Nos. 4,252,707, 4,290,929, 4,954,608, and 5,719,253 disclose the preparation of polyamic acid tertiary amine solutions. The following publications also disclose the synthesis of polyamic acid salts: R. J. W. Reynolds and J. D. Seddon, Journal of Polymer Science, Part C, Volume 23, pp45, 1968; and J. A. Kreuz, A. L. Endrey, F. P. Gay, and C. E. Sroog, Journal of Polymer Science, Part A-1, Volume 4, pp 2607, 1966; Y. Echigo, N. Miki, and I. Tomioka, Journal of Polymer Science, Polymer Chemistry, Volume 35, pp2493, 1997. U.S. Pat. No. 4,428,977 discloses the preparation of ammonium salt of partially imidized polyamic acids, which are essentially very low molecular weight oligomers.
In co-pending, commonly assigned U.S. Pat. No. 6,497,747, fabrication of polyimide membranes from precursory polyamic acid salts (PAAS) is disclosed. Specifically, said patent application discloses preparation of composite and asymmetric polyimide membranes from highly soluble PAAS polymers. Despite the intrinsic simplicity and convenience of forming polyimide membranes from PAAS precursors, the use of PAAS polymers in some embodiments can still be problematic. In particular, the evolution of large amounts of volatile components during the imidization of the PAAS polymers can have drawbacks, such as build up of residue stresses and excessive shrinkage.
One object of the present invention is to provide novel polyimide precursors, (polyimide amic acid salt (PIAAS) polymers), that exhibit improved stability, solubility and processing ability. The PIAAS polymers of the present invention include precursors for aromatic and aliphatic polyimides, preferably aromatic polyimides. The preferred PIAAS polymers have the following general structure. 
where X is a protonated tertiary amine, a quaternary amine, a phosphonium ion, a sulfonium ion, or a mixture thereof and m and n can each be in the range of 0.025 to 0.95.
Ar and Arxe2x80x2 are aliphatic or aromatic radicals. Preferably, at least 85% of said Ar and Arxe2x80x2 radicals are aromatic radicals, and most preferably 100% of said radicals are aromatic radicals.
Another object of the present invention is to provide industrially feasible processes for the manufacturing of the said polyimide amic acid salt polymers.
Still another object of the present invention is to provide a novel process for the fabrication of composite polyimide membranes.
A further object of the present invention is to provide novel, commercially viable processes for the manufacturing of asymmetric microporous polyimide membranes, including solvent resistant micro porous polyimide membranes.
A further object of the present invention is to provide novel precursory casting compositions for preparation of solvent resistant polyimide membranes. The novel precursory casting compositions are hydrolytically stable and have long shelf life.